Device for writing on thermographic material

ABSTRACT

The invention relates to a device ( 1 ) for writing on thermographic material ( 5 ). The inventive device (1) comprises a heater ( 20 ) with which the thermographic material ( 5 ) is preheated to a temperature, which is lower than a writing temperature. The thermographic material ( 5 ) can be written on with a writing instrument ( 10 ) which is distanced from the thermographic material ( 5 ) after input of an information signal (s(t)). The writing instrument ( 10 ) has a plurality of individually controllable point sources ( 30-33; 51-53 ). The thermographic material ( 5 ) can be written on in a point-by-point manner with said point sources ( 30-33; 51-53 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device for writing on thermographicmaterial. This device includes a heater for pre-heating thethermographic material to a temperature below a writing temperaturerequired for writing on the thermographic material, and a writinginstrument for writing on the thermographic material according to apredefined information signal s(t), wherein the writing instrument isspaced apart from the thermographic material.

2. Description of the Related Art

A device of this type is described in EP 0 734 870 A2. This known devicepreheats a thermographic material using a heater in form of a rotatablysupported heater drum to a temperature below a writing temperature ofthe thermographic material. The pre-heating step, however, is designedto not induce writing on the thermographic material. The light beam of asingle laser is projected on the thermographic material using an opticaldevice. The laser is modulated with an information signal. Thethermographic material includes a layer for converting radiation energyinto thermal energy. When the modulated laser beam impinges on thislayer, thermal energy corresponding to the information signal isproduced in the thermographic material. The thermal energy issuperimposed on the thermal energy produced by the pre-heating step,thereby exceeding the writing temperature of the thermographic material.The thermographic material is thereby blackened with a density variationcorresponding to the information signal modulating the laser. Theblackening of the thermographic material occurs row-by-row, with thepixels of the row being blackened consecutively. The optical deviceprojecting the laser beam on the thermographic material has a polygonmirror which rotates at a very high rotation speed. The laser beam isreflected by the polygon mirror, so that the entire row of thethermographic material can be blackened by the laser beam. The laserbeam is aimed from one end of the row of the thermographic material tothe other end. To blacken the next row of the thermographic material,the heater drum and thus also the thermographic material are rotated byanother row width.

The known device requires a complex mirror and lens arrangement forfocusing and steering the laser beam to write the entire row. Since therequired optical path is quite long, it may not be possible toaccurately steer the laser beam. Moreover, the polygon mirror must beadjusted and supported very precisely and also has to rotate at anextremely high rotation speed so that the material can be blackened in asufficiently short time.

EP 0 424 175 A2 describes a device for exposing photosensitive material.

This device has a plurality of individually addressable light emittingdiodes (LED) arranged side-by-side, so that the light sensitive materialcan be exposed pixel-by-pixel. A lens arrangement is placed between theLED's and the photosensitive material to focus the light beams emittedby the LED's. The known exposure device can eliminate intensityvariations between adjacent pixels by transmitting an identical lightenergy to the pixels in a region of the light sensitive material. LED'semit light at very low energy and can therefore not be used to write onthermographic material.

It is therefore the object of the present invention to provide a compactdevice based on conventional devices, which makes it possible to writeon thermographic material in a simple manner.

SUMMARY OF THE INVENTION

The device according to the invention for writing on thermographicmaterial includes a writing device having a plurality of individuallyaddressable point sources, wherein the point sources can be used towrite on the thermographic material pixel-by-pixel based on a specifiedinformation signal.

The invention advantageously obviates the need for a polygon mirror.Since the writing instrument is spaced apart from the thermographicmaterial, the writing instrument does not directly contact the material,thereby preventing damage and abrasion of the writing instrument as wellas of the thermographic material.

At least a portion of the individually addressable point sources can beaddressed at the same time, so that writing on the thermographicmaterial is very fast, since the points of the thermographic materialassociated with the simultaneously addressed point sources can bewritten almost simultaneously. The pixel may also be written over alonger time period, thereby increasing the response time for eachindividual point source for writing the pixel associated with the pointsource. As a result, the power to be produced by each point source forwriting the pixel associated with the point source can advantageously bekept small, because the respective point source has more time to writethe pixel. Moreover, the required response time required by therespective point source to react to a change in the signal setting canbe relatively long. Consequently, a less complex technology can be usedfor implementing the point sources.

In an advantageous embodiment of the invention, each of the individuallyaddressable point sources includes a laser. During operation of thedevice of the invention, the laser emits a laser beam which impinges onthe layer of the thermographic material which converts the radiationenergy of the laser beam to thermal energy. Advantageous, lasers providesufficiently high power and can be easily modulated with a signalsource.

Advantageously, several point sources can be connected in parallel so asto commonly write a single pixel of the thermographic material. Thepower to be produced by each of the point sources connected in parallelfor writing the associated pixel on the thermographic material canthereby be reduced according to the number of point sources connected inparallel.

According to another advantageous embodiment of the invention, means forcontrolling the radiation energy emitted by the point sources aredisposed between the writing instrument and the thermographic material.In the case where each of the point sources includes a single laser, themeans for controlling the radiation energy is simply an optical lens. Inthis way, the beam path of the emitted radiation energy of theindividual point sources can be corrected so that the radiated energy,in particular when the point sources are connected in parallel, isconcentrated in the associated pixel of the thermographic material.

In a particularly advantageous embodiment of the invention, where eachof the point sources includes one respective laser, the lasers arearranged in two rows on a semiconductor material, wherein the lasers ofone row are offset relative to the lasers in the other row. In this way,the lasers are sufficiently spaced apart during manufacture so that thesemiconductor material can be separated between two lasers. Thisapproach considerably simplifies the fabrication of the writinginstrument for a suitable number of lasers.

Advantageously, heating is provided in the form of a rotatablysupported, inductively heated drum. A first and second pressure rollercan be employed to press the thermographic material against the drum.The writing instrument is arranged so that the emitted radiation of theindividual point sources of the writing instrument impinges on thethermographic material between the two pressure rollers. By pressing thethermographic material against the drum, the thermographic material isheated during the writing step which significantly simplifies thewriting process of the thermographic material. The lasers need onlysupply a low power. In addition, the two pressure rollers can also beused to guide and advance the thermographic material.

At least one additional pressure roller may advantageously be placedbefore the first pressure roller. In this way, the thermographicmaterial is pre-heated for a longer time before being written by thelaser, so that even a relatively low heating temperature of the drumproduces a sufficiently high pre-heating temperature in thethermographic material. The thermographic material can be written veryfast due to the longer heated path traveled by the thermographicmaterial across the drum surfaces. Accordingly, the drum can be rotatedvery rapidly while still maintaining a sufficiently high pre-heattemperature.

The pressure rollers can be constructed to have a small heat capacity ormay be insulated and not absorb heat at all. With this arrangement, nothermal energy is stored in the pressure rollers and transferred to thethermographic material. Otherwise, the thermal energy stored in thepressure rollers and the thermal energy supplied by the heated drumwould be superimposed and cause unwanted blackening of the thermographicmaterial.

The point sources of the writing instrument can be easily addresseddigitally with pulse-width modulated signals. In this way, the pixelsassociated with the point sources can be written precisely on thethermographic material.

Additional advantageous embodiments of the invention are disclosed inthe dependent claims.

Other objects and features of the present invention will become apparentfrom the following detailed description considered in conjunction withthe accompanying drawings. It is to be understood, however, that thedrawings are intended solely for purposes of illustration and not as adefinition of the limits of the invention, for which reference should bemade to the appended claims.

BRIEF DESCRIPTION OF THE DRAWING

The invention and its advantages will be described hereinafter withreference to embodiments and the drawings.

It is shown in:

FIG. 1 a first embodiment of the device according to the invention forwriting on thermographic material,

FIG. 2 a second embodiment of the device according to the inventiondepicting the beam path of the point sources in operation,

FIG. 3 an arrangement of several point sources on a semiconductormaterial, and

FIG. 4 a third embodiment of the device according to the invention withseveral pressure rollers.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

In the following, the same reference numerals are used for identicalelements of the embodiments or for elements performing the samefunction.

FIG. 1 shows a first embodiment of the writing device 1 according to theinvention for writing on thermographic material 5. The writing device 1includes a laser row 10 which represents a writing instrument forwriting on the thermographic material 5 using a specified informationsignal s(t). The information signal s(t) is applied to an inputinterface 16 of a laser row controller 14 and includes information aboutan image to be recorded on the thermographic material 5. The informationsignal s(t) applied to the input interface 16 can originate, forexample, from a recording device for medical applications. Theinformation signal s(t) used to address the laser row 10 is processed inthe laser row controller 14. The laser row includes a plurality ofindividually addressable lasers which are directed towards thethermographic material 5. The laser row 10 can be used to write on a row15 of the thermographic material 5 by blackening the thermographicmaterial 5. The laser row 10 is spaced apart from the thermographicmaterial. An optical device (not shown) can be placed between theindividual lasers of the laser row 10 and the thermographic material 5for focusing the laser beams.

In the present embodiment, the individual lasers of the laser row 10 areaddressed with pulse width modulated signals. These pulse widthmodulated signals are generated by the laser row controller 14 based onthe information signal. The pulse width modulated signals generated bythe laser row controller 14 are applied to the individual lasers of thelaser row 10 via an electrical connection. The lasers are modulated withthe pulse width modulated signals and emit an intensity-modulated laserbeam in the direction of the thermographic material 5, which in thepresent example is a thermographic film. The totality of the laser beamsemitted by the laser row 10 is indicated in FIG. 1 by the referencenumeral 11. FIG. 1 depicts the laser beam 12 of the outermost rightlaser and the laser beam 13 of the outermost left laser of the laser row10.

The writing device 1 according to the invention illustrated in FIG. 1includes a heater in the form of a rotatably supported, inductivelyheatable drum 20. In this way, the temperature of the drum 20 can becontrolled almost without dead time, and a relatively small drum 20having a small heat capacity can be used. However, a different heatabledrum may also be employed.

The drum 20 can be rotated in a rotation direction A and is connected toa drum heater controller 23 capable of controlling the temperature towhich the drum 20 is heated. The drum 20 is located directly underneaththe laser row 10. The thermographic film 5 can be brought into contactwith the drum 20 between the laser row 10 and the drum 20. For improvingthe contact and for guiding and transporting the thermographic film 5,the writing device 1 has two pressure rollers 21 and a 22 which arearranged between the laser row 10 and the drum 20 in such a way that thethermographic film 5 can be driven, on one hand, between the pressurerollers 21 and 22 and, on the other hand, the drum 20. The pressureroller 21 is disposed before row 15 of the thermographic film 5 whereasthe pressure roller 22 is disposed after row 15. The two pressurerollers 21 and 22 press the thermographic film 5 against the drum 20,enabling the thermal energy emitted by the drum 20 to heat thethermographic material before and during the writing process. Thethermographic film 5 is then transported to a feed device B.

The pre-heating time is determined by the rotation speed of the drum 20and the spacing between the contact point of the first pressure roller21 on a heated drum 20 and the location where the pixels are written onthe thermographic film 5. The thermographic film 5 typically requiresbetween 0.3 and 0.5 seconds to reach the temperature of the drum 20. Thetemperature of the drum 20 is advantageously between 110 and 115° C. Thetemperature has to be lower than the temperature required to write onthe thermographic material 5; the temperature of the drum 20 is specificfor the respective thermographic material. Pre-heating of thethermographic film 5 should not cause the film 5 to fog. However, thehigher the selected pre-heating temperature for the film 5, the morepower has to be supplied by the individual lasers of the laser row 10 towrite on the film 5. It is therefore advantageous to precisely controlthe temperature of the drum 20 and to match the temperature to thespecific selected thermographic film material.

The pressure rollers 21 and 22 in the present embodiment have a verysmall heat capacity, so as to limit the amount of thermal energy storedin the pressure rollers. In this way, the thermal energy stored in thepressure rollers cannot affect the writing process of the thermographicfilm 5. Alternatively or in addition, the same effect can be achieved byinsulating the pressure rollers 21 and 22 so that they do not absorbheat.

Advantageously, the second pressure roller 22 can also be arranged sothat the portion of the thermographic film 5 which has already beenwritten, is removed very quickly from the circumferential surface of thedrum 20. This prevents additional heating of the written portion of thefilm 5 which could otherwise cause additional unwanted blackening of thefilm 5. Accordingly, as indicated in FIG. 1, the spacing between theheating drum 20 and the first pressure roller 21 is smaller than thespacing between the heating drum 20 and the second pressure roller 22.However, the thermographic film 5 still has to be precisely guided, inparticular to prevent the film 5 from buckling at the location of therow 15 which is to be written.

In the present embodiment, the thermographic film 5 has a width of 14″(=355.6 mm). Other film widths, for example, a width of 8″ or 17″, canalso be used. A pixel of the thermographic film 5 has a fixed width of80 μm, providing a resolution of 300 dpi. Two adjacent lasers of thelaser row 10 then advantageously also have a fixed center-to-centerspacing of 80 μm. A total of 4256 lasers are provided in the laser row10 for writing a row of the thermographic film 5. A respective pixel ofthe row 15 of the thermographic film 5 is associated with each of these4256 lasers.

The operation of the writing device 1 according to the invention willnow be described. The information signal s(t) is applied to the laserrow controller 14 at the input interface 16. The information signal s(t)contains information which is to be imaged on the thermographic film 5.The laser row controller 14 which controls the laser row 10 processesthe information signal s(t) and produces a signal for addressing eachlaser of the laser row 10. In other words, in the present exemplaryembodiment, 4256 signals are generated from the information signal s(t).The addressing signals generated by the laser row controller 14 directlymodulate the individual lasers of the laser row 10. In the presentexemplary embodiment, the individual lasers are controlled digitally byusing, for example, pulse width modulated control signals. Thisarrangements allows the pixels of the thermographic film 5 associatedwith the individual lasers to be written with particular precision. Theexposure duration of the pixels associated with the individual lasersdetermines the degree of blackening of the respective pixels. In thisway, different gray levels can be is produced on the thermographic film5. The digital control of the lasers represents an advantageousembodiment of the invention. It will be understood that the lasers canalso be addressed in an analog manner. Processing the information signals(t) in the laser row controller 14 is not part of the invention and canbe adapted by those skilled in the art to the specific conditions.

In the present exemplary embodiment, the control signals produced by thelaser row controller 14 address the lasers of the laser row 10simultaneously. This arrangement allows the pixels of the row 15 of thethermographic material 5 to be written simultaneously. A row canadvantageously be written on the thermographic material 5 by the laserrow 10 in approximately 3 ms. This allows the thermographic film 5 to becompletely written within a very short time.

Different rows can be written consecutively on the thermographic film 5by rotating the heating drum 20 in the rotation direction A. The drumrotation is continuous, thereby obviating the need for a complexstepping motor to drive the heating drum 20. Accordingly, thethermographic film 5 is advanced in the feed direction B by the rotationof the heating drum 20.

The radiation energy of the laser beams of the individual lasers isconverted by a particular layer disposed in the thermographic film 5when the laser beam impinges on the thermographic film 5. The amount ofthe thermal energy depends on the intensity of the laser beam and theexposure duration. Since the laser in the present exemplary embodimentare addressed with pulse width modulated signals, the intensity of thelaser beam can ideally only assume two states. The intensity of thelaser beams is either equal to zero or equal to a maximum value whichdepends on the predetermined maximum output power of the individuallasers. The pixels of the film 5 associated with the individual lasersassume different optical densities depending on the produced thermalenergy. Since the individual lasers are controlled digitally, thedifferent densities (blackening) of the pixels of the film 5 depend onthe exposure duration of the individual pixels.

A different radiation source can be used instead of the laser row 10,wherein the radiation source is composed of a plurality of individuallyaddressable radiation sub-sources. It should be noted, however, that theoutput power of these radiation sub-sources should be high enough toblacken the pre-heated thermographic material, producing differentdensity gradations. Alternatively, individually addressable heat sourcesmay be used instead of the plurality of individually addressableradiation sub-sources. These individually addressable heat sources couldthen augment the thermal energy produced by pre-heating to generate thethermal energy necessary to write on the thermographic material. Thisarrangement would obviate the need of a special layer which converts theradiation energy into thermal energy in the thermographic material 5.

The writing device 1 of FIG. 1 is constructed so that the laser row 10has as many lasers as are required to simultaneously write the pixels ofan entire row of the thermographic film 5. One respective laser of thelaser row 10 is here associated with one respective pixel. The laser rowcontroller 14 converts the information signal s(t) into as many pulsewidth modulated signals as there are lasers in the laser row 10.Alternatively, the laser row controller 14 many generate a lesser numberof control signals than there are lasers in the laser row 10. In thiscase, only a portion of the lasers can be addressed simultaneously bythese control signals. The complete row 15 of the film 5 would then haveto be written, for example, in two or more steps. Alternatively, thelaser row 10 can also have a lesser number of lasers than there arepixels in a row of the film 5. In this case, the writing deviceaccording to the invention would have to move the thermographic film 5relative to the laser row 10 along the direction of one of the rows ofthe film 5.

FIG. 2 shows a second embodiment of the device according to theinvention depicting the beam path of the point sources in operation. Inthis example, the point sources are also lasers. FIG. 2 shows a sectionof the laser row 10 with four lasers 30-33 arranged side-by-side. Thelasers 30 to 33 are shown in operation, when emitting a respective laserbeam 41-44. The laser beams 41-44 in the present exemplary embodimentare directed perpendicular to the thermographic film 5 to be written. Alens 40 is disposed between the lasers 30-33 and the film 5. This lens40 is a commercially available so-called SELFOC lens. The optical lens40 is used to affect the radiation energies of the beam path 41-44 ofthe lasers 30 to 33 and to ensure that the laser beams of lasers 30-33strike the thermographic film 5 exactly at the associated pixels. Thebeam path 41-44 of the lasers 30-33 is therefore converted by theoptical lens 40 into the beam path 45-48 located between the lens 40 andthe film 5. The lens should be able to focus or to defocus the beampaths depending on the characteristic features of the lasers 30-33 andthe beam shape of the laser radiation 41-44 produced by these lasers.

FIG. 3 shows a section consisting of an arrangement of several pointsources, which in this exemplary embodiment are lasers, disposed on asemiconductor material. FIG. 3 depicts a plurality of lasers which arearranged on a semiconductor wafer 50, representing a portion of a laserrow for writing on thermographic material. The lasers are arranged ingroups, with each group having three sub-lasers. The sub-lasers of agroup are connected in parallel and are used to simultaneously write apixel of the thermographic material. A group of sub-lasers isrepresented in FIG. 3 showing a first sub-laser 51, a second sub-laser52 and a third sub-laser 53. The control terminals of the threesub-lasers 51-53 are connected to the laser row controller 14 with abonding wire 54. The pulse width modulated control signals are appliedto the control terminals of the three sub-lasers 51-53 via this bondingwire 54. Since the three sub-lasers 51-53 are connected in parallel,they emit identical intensity modulated laser beams. Moreover, withseveral sub-lasers (in this exemplary embodiment the three sub-lasers51-53) being arranged in parallel, the radiation energies of the threesub-lasers are superimposed in the associated pixel of the thermographicfilm 5. In this way, the output power of the individual sub-lasers canbe kept small, while still maintaining a correspondingly high thermalenergy for blackening of the film 5.

In the present exemplary embodiment illustrated in FIG. 3, the groups ofsub-lasers are arranged side-by-side in two rows 55 and 56. The groupedsub-lasers of the first row 55 are offset with respect to the groupedsub-lasers of the second row 56. This arrangement produces channels onthe semiconductor wafer between the individual laser groups; thesechannels can be used for cutting the semiconductor wafer when the laserrow is fabricated. FIG. 3 shows such a channel 57 on the semiconductorwafer. An arrangement where the groups of sub-lasers are offset withrespect to one another, is advantageous because during the fabricationof the laser rows many groups of sub-lasers are produced simultaneouslyon a semiconductor wafer, with the sub-lasers subsequently separated bysawing, scribing and cleaving. The saw cuts or cleaves should not belocated too closely to the active structures of the laser row so as notto damage the lasers. The laser row controller 14 has to take the offsetarrangement of the groups of sub-lasers into consideration whengenerating the pulse width modulated control signals. Accordingly,corresponding controls have to be implemented in the laser rowcontroller 14.

FIG. 4 shows a third embodiment of the device according to the inventionhaving several pressure rollers which press the thermographic film 5against the heating drum 20. According to FIG. 4, a third pressureroller 24 and a fourth pressure roller 25 are placed before the firstpressure roller 21. Pre-heating the film 5 with several pressure rollers21, 24 and 25 increases the length of the contact path between thethermographic film 5 and a surface of the heating drum 20 andconsequently also the time during which the heating drum 20 pre-heatsthe thermographic film 5. In this way, the pre-heating conditions forthermographic material 5 can be tailored to different thermographicmaterial. The duration of the pre-heating step can be extended orshortened depending on the composition and the characteristic propertiesof the different thermographic materials. In addition, the thermographicmaterial 5 can be pre-heated more accurately to a temperature below thewriting temperature.

Thus, while there have been shown and described and pointed outfundamental novel features of the invention as applied to a preferredembodiment thereof, it will be understood that various omissions andsubstitutions and changes in the form and details of the devicesillustrated, and in their operation, may be made by those skilled in theart without departing from the spirit of the invention. For example, itis expressly intended that all combinations of those elements and/ormethod steps which perform substantially the same function insubstantially the same way to achieve the same results are within thescope of the invention. Substitutions of elements from one describedembodiment to another are also fully intended and contemplated. It isalso to be understood that the drawings are not necessarily drawn toscale but that they are merely conceptual in nature. It is theintention, therefore, to be limited only as indicated by the scope ofthe claims appended hereto.

What is claimed is:
 1. A device for writing on thermographic material comprising a heater for pre-heating the thermographic material to a temperature below a writing temperature required for writing on the thermographic material; a writing instrument for writing on the thermographic material according to a predefined information signal s(t), wherein the writing instrument is spaced apart from the thermographic material; wherein the writing instrument comprises a plurality of individually addressable point sources capable of writing on the thermographic material point-by-point; wherein the plurality of individually addressable point sources are arranged in such a way that pixels of a row of the thermographic material can be written; wherein the plurality of individually addressable point sources can be addressed in such a way that the pixels of a row to be written by the point sources can be written simultaneously; wherein the plurality of individually addressable point sources are implemented so as to be capable of emitting a radiation which impinges on a layer of the thermographic material which is provided for converting the radiation into heat; wherein the point sources comprise lasers; and wherein several of the plurality of individually addressable point sources are connected in parallel and in combination are capable of writing a single pixel of the thermographic material.
 2. A device for writing on thermographic material comprising a heater for pre-heating the thermographic material to a temperature below a writing temperature required for writing on the thermographic material; a writing instrument for writing on the thermographic material according to a predetermined information signal s(t); the writing instrument being spaced apart from the thermographic material, and comprises a plurality of individually addressable point sources capable of writing on the thermographic material point-by-point; the plurality of individually addressable point sources being arranged in such a way that pixels of a row of the thermographic material are capable of being written; the plurality of individually addressable point sources being addressable in such a way that the pixels of a row to be written by the point sources is adapted to be written simultaneously; wherein the plurality of individually addressable point sources are implemented so as to be capable of emitting a radiation which impinges on a layer of the thermographic material which is provided for converting the radiation into heat; and wherein the point sources comprise lasers arranged on a semiconductor material in two rows, the lasers of the one row being offset with respect to the lasers of the other row.
 3. The device according to claim 2, wherein between the writing instrument and the thermographic material there is arranged a means for affecting the radiation emitted by the point sources to support point-by-point writing on the thermographic material.
 4. The device according to claim 3, wherein the means for affecting the radiation is an optical lens.
 5. The device according to claim 4, wherein the heater is a rotatably supported, heatable drum.
 6. The device according to claim 5, further comprising a first pressure roller and a second pressure roller for pressing the thermographic material against the drum and the writing instrument is arranged so that the thermographic material can be written between the two pressure rollers.
 7. The device according to claim 6, wherein at least one additional pressure roller is disposed after the first pressure roller.
 8. The device according to claim 7, wherein the first and the second pressure rollers have a small heat capacity or are insulated to prevent absorption of heat.
 9. The device according to claim 8, wherein there is provided a controller connected to the point sources, wherein the controller converts the information signal (s(t)) into a plurality of pulse width modulated signals. 